Understanding effect of structure and stability on transformation of CH4 hydrate to CO₂ hydrate

Highlights: • CH₄ molecules are likely to reside in 5¹² cages rather than 5¹²6² cages. • CO₂ molecules are likely to reside in 5¹²6² cages rather than 5¹² cages. • CO₂ has smaller interaction energy than CH₄ in the same cage. • The entropy dominates the transformation of CH₄ hydrate into CO₂ hydrate. • In the same cage CO₂ molecules move much slowly than CH₄ molecules. Understanding the transformation process of CH₄ hydrate to CO₂ hydrate is crucial to develop the CH₄CO₂ replacement technique for CH₄ production and CO₂ sequestration. Ab initio calculations show that the transformation will slightly distort the host lattice and decrease the binding strength of guest molecules, but it is a thermodynamically spontaneous process dominated by the entropic contribution. Moreover, ab initio molecular dynamics simulations suggest that the dynamics of the host lattice is independent on the guest molecules, while CO₂ in hydrate exhibits slower translational and rotational motion than CH₄ in hydrate.